The easiest way to do this is to put a bunch of invisible sprites - SpawnPoints.

Then you can do something like this:

Every 2 seconds
Repeat round(random(10,15)) times
System-> Pick random instance of SpawnPoint : SpawnPoint spawn Enemy
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This code is not very good because the same SpawnPoint may be selected several times and spawned enemies will overlap.
A better solution would be adding Timer behavior to SpawnPoint. Each SpawnPoint instance will be running its own timer and spawning enemies on timer event.
You can also add a condition to check if there are other enemies within X pixels from the SpawnPoint before spawning a new enemy.